Several types of dry powder inhalers are known and in this respect reference should be made to WO 97/40876, WO 98/41256 and WO 92/04069 which disclose examples of such inhalers.
WO 97/40876 describes an inhalation device comprising a suction tube and blister pack assembly. The blister pack assembly is in the form of a carrier or support unit which holds the blister pack. The carrier or support unit is configured such that the upper surface has a plurality of holes which sit above the blisters in the blister pack. Accordingly, the distal end of the suction tube can be placed in a blister when the user needs to inhale the powdered medicament.
WO 98/41256 describes an inhalation device which is known as the TURBUHALER®. This inhalation device has a dosing means which is operated by twisting a rotatable gripping portion. The twisting action releases a dose of powdered medicament into a dosing unit which can then be inhaled by the user.
WO 92/04069 describes an inhalation device which is known as the MONOHALER® delivering only a single dose of powdered medicament. The powdered medicament is released by removing sealing foil portions and is then simply inhaled by the user inhaling through the mouthpiece.
Dry powder inhalers (DPI's) have many advantages over inhalers such as pressurised metered dose inhalers (PMDI's). For example, no propellants are needed, pure drug administration is possible and they are relatively simple to operate. However, a disadvantage encountered in dry powder inhalers is their sensitivity to moisture. Some dry powder formulations suffer negative effects when the humidity in the air inhaled through the inhalation device increases. In particular, the relative humidity (RH) will result in an increase in retention of the powder formulation in the inhalation device. At high relative humidity the water molecules in the humid air will react with the surface of the particles of the powder formulation during the short time it takes for the incoming inhaled air to move or lift and deaggregate the powder.